1. Field of the Invention
This invention relates to optical fibers and, in particular, to coated optical fibers.
2. Art Background
Optical fibers are generally drawn from a glass body (typically denominated a preform) and coated with a protective organic coating in one continuous process. This continuous process is accomplished with an apparatus (a drawing tower). The fiber is initially drawn from the preform and then coated by passing it through a reservoir of organic material. Organic liquids which are UV-curable (e.g., acrylated resins such as epoxy acrylates and urethane acrylates) or thermally curable, e.g., silicones, are often employed so that the complete formation of the final coating is conveniently accomplished merely by irradiation or heating.
The coating procedure -- not the drawing procedure -- usually limits the maximum rate at which an acceptable, coated fiber is produced. Since a large capital investment is involved in a drawing tower, it is naturally desirable to run the drawing and coating process at the highest speed that is consistent with the production of a coating of acceptable quality. In the manufacture of 125 .mu.m in diameter fiber utilizing typical coating procedures, e.g., where a UV-curable urethane-acrylate coating and where a distance of 1.25 m between drawing and coating points is employed, drawing speeds of approximately 1.3 m/sec are employed. At faster speeds coating irregularities such as beads, thickness non-uniformities and in severe cases intermittent coatings typically occur. Generally, to ensure desirable transmission performance in a fiber cable--a body containing a multiplicity of fibers--the coating outside diameter should not vary more than .+-.10 percent. A significant parameter affecting the development of coating irregularities is the temperature of the fiber entering the coating liquid. It is believed that if the fiber is not sufficiently cool the coating material does not form a continuous film of acceptable uniformity. Thus the faster the coating rate the shorter the time for cooling and the lower the likelihood of forming a suitable coating.
Various techniques have been investigated to allow more rapid drawing and coating speeds while maintaining coating quality. The most established technique, which allows a desirable increase in speed, involves increasing the distance between the point the fiber is drawn and the point of entry of the fiber into the coating liquid yielding more time for cooling. However, this increased distance requires a significantly larger drawing apparatus and a concomitant increase in capital investment.
Other techniques directed to increasing coating speed have been investigated. For example, an air flow has been directed onto the fiber after it is drawn, but before it is coated. Two significant adverse consequences have resulted from this procedure. An air flow sufficient to allow significant speed increase also causes substantial fiber vibration. This vibration, in turn, produces unacceptable variations in fiber diameter. (An acceptable variation is a standard deviation over the length of the fiber of less than 1%, preferably less than 0.25%.) Additionally, it has been reported that the use of air degrades the mechanical properties of the fiber. (See Paek and Schroeder, Applied Optics, Vol. 20, 1981.) The use of helium as a coolant rather than air has also been suggested. (See European Patent application No. 82305708.8, filed Jan. 11, 1982, having a publication number 8079186.) Helium injected in a radial direction relative to the fiber was reported to increase fiber coating speed.
Although quite useful drawing speeds have been attained, significantly higher speeds produce corresponding economies.